The present invention relates to an apparatus for detecting a focus condition of an imaging lens of an optical machine such as a still camera, cinecamera, video camera and microscope.
There have been proposed various focus detection systems. In one of the known systems the focus condition is judged by detecting the amount and direction of a lateral shift of two images formed by two light fluxes transmitted through different portions of an imaging lens. The present invention relates particularly to a focus detection apparatus for such a lateral shift detection system.
FIG. 1 is a schematic view showing an optical system of the focus detection apparatus of a lateral shift detection system. An exit pupil of an imaging lens 1 is divided into sections by means of a pupil dividing means 2 such as a stripe mask, a micro lens array and a micro prism array.
The divided pupil images are projected upon a light receiving element array 3 arranged in a predetermined focal plane or a plane conjugated therewith. It should be noted that the light receiving element array 3 may be arranged in the proximity of these planes.
The exit pupil images of the imaging lens 1 divided by the pupil dividing means 2 are projected on adjacent paired light receiving elements of the array 3 such that the divided images of the exit pupil of imaging lens 1 projected on first elements 3A-1, 3A-2 . . . 3A-n and those projected on corresponding second elements 3B-1, 3B-2 . . . 3B-n become coincident with each other in an in-focus condition, but are shifted in opposite directions in accordance with a direction of a deviation from the in-focus condition. Then, the focus condition is detected by processing outputs of the odd and even numbered light receiving elements.
In the known focus detection apparatus of a lateral shift detection system, the focus condition cannot be detected precisely due to image height. This will be explained in detail hereinbelow.
As illustrated in FIG. 2, it is now assumed that a distance between an exit pupil plane 6 of the imaging lens 1 and an image plane 5 is Lo, and a distance between an image point 5-1 of a point 4-1 of an object 4 and an optical axis 7 is x. The distance x is sometimes called the image height. Then, since a principal light ray 8 from the image point 4-1 is inclined with respect to the optical axis 7 by an angle .alpha.(x)=tan.sup.-1 (x/Lo), the division rate of the exit pupil by means of the pupil dividing means 2 with respect to the paired light receiving elements varies dependent upon the distance x, i.e., the image height. Therefore, in the known focus detection apparatus, the focus condition could not be detected accurately.
Now the undesired influence of the image height will be further explained in detail with reference to FIG. 3.
In the known focus detection apparatus shown in FIG. 3, the exit pupil dividing means 2 comprises a stripe mask which includes light transmitting portions separated from each other by a pitch 2P. The light receiving element array 3 comprises light receiving elements spaced apart from each other by a pitch P. In FIG. 3, the light transmitting portions 2i, 2j of the stripe mask 2 and the light receiving elements 3A-i, 3B-i corresponding to the portion 2i and elements 3A-j and 3B-j corresponding to the portion 2j are shown for the sake of clearness. The paired elements 3A-i, 3B-i and 3A-j, 3B-j receive light fluxes transmitted mainly through different sections of the exit pupil of the imaging lens 1.
Now central points 2S-i and 2S-j of the light transmitting portions 2i and 2j are considered. The paired light receiving elements 3A-i and 3B-i on the optical axis 7 receive the images of the exit pupil of imaging lens 1 which are equally divided with respect to the central point 2S-i of the relevant light transmitting portion 2-i of the stripe mask 2. Contrary to this, the paired light receiving elements 3A-j and 3B-j receive exit pupil images which are not equally divided with respect to the central point 2S-j of the related light transmitting portion 2-j. That is to say, in accordance with the increase in the image height, one of the paired light receiving elements 3B-j receives a large light flux transmitted through a substantial portion of the exit pupil of imaging lens 1, but the other light receiving element 3A-j receives a small light flux transmitted through only a peripheral portion of the exit pupil of the imaging lens. Therefore, even when a wholly white object is imaged, outputs of the light receiving elements have a non-uniform distribution shown in FIG. 4. The outputs Ai and Bi are supplied from the elements 3A-i and 3B-i, respectively, corresponding to the light transmitting portion 2i having the central point 2S-i on the optical axis 7, and become equal to each other. However, the outputs A.sub.1 and B.sub.1 from the elements 3A-1 and 3B-1, respectively, and the outputs An and Bn from the elements 3A-n and 3B-n, respectively, corresponding to the light transmitting portions spaced apart from the optical axis 7, become different from each other. That is to say, the outputs A.sub.1, A.sub.2 . . . An of the odd numbered elements are gradually decreased, and the outputs B.sub.1, B.sub.2 . . . Bn of the even numbered elements are gradually increased.
Due to the above explained imbalance of the division rate, when the object has a step-like contrast as shown in FIG. 5A, envelopes of outputs from the odd and even numbered elements are not equal to each other, but are different from each other as illustrated by a broken curve A and a solid curve B in FIG. 5B. In this case, the apparatus could not detect the infocus condition accurately and thus the focus detection precision is low.
In order to eliminate the above mentioned problem, it has been proposed in Japanese Patent Application Laid-open Publication No. 130,524/80 that a correction lens be arranged between the imaging lens and the exit pupil dividing means to correct the inclination of the principle light ray of imaging lens due to the image height. FIG. 6 is a schematic view showing an optical system in which the correction lens is applied to the focus detection apparatus illustrated in FIG. 3. Between the imaging lens 1 and the stripe mask 2 a correction lens 9 is inserted for converting the light flux transmitted through the imaging lens 1 into a parallel light flux to correct the inclination of the principal light ray of the light flux due to the image height. However, this known solution has another drawback in that the correction lens is required in addition to the pupil dividing means, making the whole apparatus larger and more expensive. Also, the adjustment during manufacture becomes complicated and cumbersome.